1. Field of the Invention
The invention relates to the field of manufacture of microelectronics devices employing patterned etch mask resist layers to form patterns. More particularly the invention relates to the employment of electron beam microlithography fabrication methods to form patterned etch resist mask layers and patterned photomasks.
2. Description of the Related Art
Microelectronics fabrications consist of multiple layers of microelectronics materials formed on a substrate. Many of the microelectronics layers are patterned and must be not only accurate and precise in themselves, but must also be registered with great precision to other patterned layers. These objectives are met by employing photolithographic methods in which the desired patterns are first formed as patterned stencil photomasks with opaque and transparent regions. This pattern is then transferred to a photosensitive layer or photoresist by illumination through the patterned photomask, causing a chemical difference in the illuminated and non-illuminated regions. This difference may be exploited by subsequent chemical development of the pattern image in the photoresist layer which is then employed as a patterned photomask layer for fabrication purposes such as, for example, subtractive etching of an underlying material layer to transfer the pattern.
In order to fabricate photomasks for pattern formation in microelectronics fabrication, it is necessary to start with a master image of the desired pattern. In the early stages of development of microelectronics fabrication technology, such master image patterns were generally enlarged versions of the pattern which were then reduced by photographic methods to the final dimensions on the working photomask This process was tedious and costly, and has been largely replaced by the use of direct exposure of the photomask opaque blank substrate, coated with a layer of sensitive resist material, to the desired pattern. The exposure is normally done with a directed electron beam to obtain the required precision and fine dimensions required. The energy absorbed from the directed electron beam is integrated by the resist material layer into a chemical change which can be exploited to develop the exposed pattern in the resist layer such that the pattern can then be transferred into the underlying opaque substrate layer.
Because the resist layer integrates all energy to which it is exposed, not only is the directly incident electron beam energy stored in the resist, but also any stray electrons from scattering processes elsewhere are capable of registering their effect on the resist. Thus the total energy absorbed by a given exposed region is not only a function of the electron energy dose intentionally delivered by design to that region, but also a function of the electrons absorbed from those delivered nearby and scattered back into the intended exposed region. This so-called “proximity effect” on the actual energy dose absorbed by the resist layer in a given region from the design nominal electron dose delivered to a region and that absorbed from electrons from nearby regions due to scattering is a significant effect on the accuracy with which the developed resist image follows the designed pattern of electron beam energy delivery. The proximity effect may be divided into “mutual proximity ” effects from nearby electrons scattered sideways from adjacent pattern elements, and “self-proximity” effects from electrons delivered directly to the desired region which after passage through the resist layer and into the substrate are fortuitously scattered backwards at lower energy.
The correction for proximity effects to improve on the accuracy of electron beam exposed resist patterns is generally accomplished by adjustment of the actual electron energy dose delivered for exposure after taking into account the pattern of nearby exposures and estimating the degree of extra electron energy from scattering, and reducing the delivered dose accordingly. Although effective for many purposes, this dose correction method is not without problems, particularly with respect to being costly and time-consuming.
Another method for improving the accuracy of electron beam exposure of resist layers is known as the “ghost” correction method. In this method, the desired pattern of exposed resist is written in two steps: a first pattern which is the desired pattern written at a fixed dose, and a second pattern which is a negative reversal of the first pattern and written at a lower dose, generally with a defocused electron beam. The method relies on the total dose at the edge of a first pattern feature to have its slightly lower actual dose increased by the background exposure dose of the second pattern exposure to provide the desired pattern exposure dose for proper pattern image development.
Although the method of dose correction of the written pattern or the “ghost” correction method are in general satisfactory for general use in electron beam lithography, neither method is entirely without problems. Densely populated patterns require inordinately long and costly calculation of incremental dose correction adjustments for each pattern element in the dose correction method. For small features and/or sparsely populated designs, the time required for the second exposure of the “ghost” correction method is time consuming and the defocused beam may cause resolution problems.
It is thus towards the goal of forming patterned resist mask layers and/or photomasks by irradiation of sensitive material layers employing electron beam lithography with correction of proximity effects to improve pattern accuracy that the present invention is generally directed.
Various methods have been disclosed for the formation of mask layers and masks by electron beam pattern generation with correction for proximity effects.
For example, Abe et al., in U.S. Pat. No. 5,451,487, disclose a method for correction of electron beam exposure of patterns based on the “ghost” method which greatly decreases the time for correction. The method calculates a dose required for a representative figure combining a number of smaller pattern features, and then supplies the required dose employing a defocused beam to write the inverted pattern.
Further, Pan et al., in U.S. Pat. No. 5,510,214, disclose a method for forming a double destruction phase shift mask (PSM) which eliminates the spurious “ghost” line in the mask image which may occur in conventional phase shift masks. The method combines transparent phase shifting regions with attenuating phase shifting regions to form interference patterns which reduce the light intensity transmitted to nearly zero in the pattern elements of the mask.
Still further, Ham, in U.S. Pat. No. 5,582,938, discloses a method for forming a phase shift mask which prevents the formation of a “ghost” image due to interference and diffraction of light with a phase angle of 0 and 180 degrees. The method employs a photoresist layer.
Yet still further, Veneklasen et al., in U.S. Pat. No. 5,847,959, disclose a method for correcting an electron beam pattern for proximity effects due to electron scattering, heating and thermal expansion effects. The method employs a raster scanning electron beam in which calculated corrections for the various proximity effects are applied to the delivered dose as correction factors.
Finally, Ohnuma, in U.S. Pat. No. 5,885,748, discloses a method for correcting photomask patterns for proximity effects due to electron beam scattering or light exposure employing the photomask. The method utilizes correction of the pattern by forming a mesh and determining if another portion of the pattern is close enough to cause a proximity effect. If so, the dose is corrected to result in a final exposure pattern which is close to the design pattern.
Desirable in the art of microelectronics fabrication are further methods for forming patterned resist mask layers, bipolar photomasks and phase shift photomasks with electron beam exposure of patterns with correction for proximity effects.
It is towards these goals that the present invention is generally and more specifically directed.